1. Field of the Invention
The present invention relates to new plants of the family Compositae, in particular lettuce plants of the genus Lactuca which are resistant to the aphid Nasonovia ribisnigri and herein display agronomically desirable traits.
2. Description of the Related Art
Aphids cause much damage in vegetable cropping. They feed on the phloem of the plants and thereby cause reduced or abnormal growth. Living aphids or aphid remnants make the harvested product unsaleable. In addition, honeydew, a sugary liquid secreted by the aphids, forms a sticky layer on the leaves. Aphids are greatly feared, not only because of this direct damage, but also because they spread virus diseases.
Nasonovia ribisnegri is the aphid species which in Europe is found most frequent on lettuce grown in the field. N. ribisnigri is particularly harmful because it prefers to feed on the young leaves of the plant. The aphids hereby become easily trapped in the closing lettuce heads, making them difficult to reach for pesticides. Especially in the crisphead types of lettuce, which form a tight head, this causes great problems. Crop farmers limit the damage caused by aphids by repeatedly spraying pesticides on the growing crops, in particular when the weather conditions are favourable for the reproduction of aphids. The use of excessive quantities of pesticide is undesirable from an environmental viewpoint. Pesticides moreover miss their target in particular cases, as described above for instance in the case of closing lettuce heads.
Resistance of the plants on which aphids occur (host plant resistance) is an environmentally-friendly alternative for the use of pesticides to control the development of aphids in for instance lettuce. Therefore extensive research has already been done into resistance to N. ribisnigri and into the inheritance of this type of resistance. For a number of lettuce cultivars a partial resistance to N. ribisnigri was described (Dunn, J. A. & Kempton, D. P. H.(1980) Tests of Agrochemical and Cultivars, No. 1, (Ann, appl. Biol. 94, Supplement): 58-59). An almost complete resistance to N. ribisnigri was found in the wild Lactuca variety L. virosa L. (Eenink A. H. & F. L. Dieleman (1983) Euphytica 32:691-695). This Nasonovia resistance in L. virosa was found to be caused by a single dominant gene, which is called Nr gene.
In 1981 L. sativa plants were released by the former Institute for Horticultural Plant Breeding (IVT, now part of CPRO-DLO) in Wageningen which contained in their genome an L. virosa chromosome fragment having the Nr gene for resistance to N. ribisnigri. These plants resulted from a hybridization program with L. virosa and L. sativa, in which L. serriola was used as bridge species. A bridge species is used when two species can only be crossed with one another to a limited extent, as was the case with L. virosa and L. sativa.
The released plants were of an undesirable type in respect of phenotype and agronomic traits (non-heading, poor cultivation characteristics).
Because of the undesirable agronomic traits these plants were used by breeding companies as hybridization parent for the purpose of obtaining plants by genetic recombination and selection, which combine resistance to N. ribisnigri with good agronomic traits.
L. sativa is an annual species but, when it is cultivated under artificial light at an increased temperature, 2-5 successive generations can be produced within a year. Backcrossing procedures are a generally known and suitable method for crossing genes from a "donor parent" into a genetic background with a high agronomic value. In general the introgression of a dominant gene into an agronomically acceptable phenotype can be achieved by 3-5 backcrosses, followed by 2-3 self-pollinations. 5-8 generations in 3-5 years are therefore required to obtain agronomically acceptable plants having the desired gene in their genome. However, although plants having the Nr gene were already released in 1981 to seed companies, the successful transfer of the Nr gene to agronomically acceptable lettuce plants has so far not yet been reported.
It is now the object of the present invention to provide plants of the family Compositae, and in particular new lettuce plants, which combine resistance to aphids of the species Nasonovia ribisnigri with agronomically good traits.
This object is achieved according to the invention in that it was found in the course of a selection program that the application of this resistance in lettuce plants having good agronomic characteristics was prevented by negative side-effects caused by the L. virosa chromosome fragment, on which the Nr gene was situated, when it was inserted in an L. sativa genome. Compared with cultivated lettuce plants without the N. ribisnigri resistance, plants which were homozygous for the Nr gene in fact displayed a reduced growth, a lighter green color and accelerated degradation of chlorophyll in the older leaves of the generative plant (when the plant is bolting). This resulted in generative plants having completely or partially white older leaves and/or reduced plant height. This agronomically undesired phenotype will be further designated in this application as "CRA phenotype", meaning "Compact growth and Rapid Ageing". FIG. 1 gives an example of the difference between normal lettuce plants and CRA plants. In the vegetative stage the CRA symptoms are particularly apparent when the plants grow under stress (for instance at low temperature).